1. Field of the Invention
The present invention relates to surgical apparatus and more particularly to a neurophysiological monitoring system including a nerve integrity monitoring instrument for use in conjunction with one or more electrical stimulus probes as an intraoperative aid in defining the course of neural structures. The invention is particularly applicable for use in monitoring facial electromyographic (EMG) activity during surgeries in which a facial motor nerve is at risk due to unintentional manipulation, although it will be appreciated that the invention has broader applications and can be used in other neural monitoring procedures.
2. Discussion of the Prior Art
Despite advances in diagnosis, microsurgical techniques, and neurotological techniques enabling more positive anatomical identification of facial nerves, loss of facial nerve function following head and neck surgery such as acoustic neuroma resection is a significant risk. Nerves are very delicate and even the best and most experienced surgeons, using the most sophisticated equipment known, encounter a considerable hazard that a nerve will be bruised, stretched or severed during an operation. Studies have shown that preservation of the facial nerve during acoustic neuroma resection may be enhanced by the use of intraoperative electrical stimulation to assist in locating nerves. Very broadly stated, the locating procedure, also known as nerve integrity monitoring, involves inserting sensing or recording electrodes directly within cranial muscles enervated or controlled by the nerve of interest. A suitable monitoring electrode is disclosed in U.S. Pat. No. 5,161,533 (to Richard L. Prass et al.), the entire disclosure of which is incorporated herein by reference.
One method of nerve localization involves the application of electrical stimulation near the area where the subject nerve is believed to be located. If the stimulation probe contacts or is reasonably near the nerve, the stimulation signal applied to the nerve is transmitted through the nerve to excite the related muscle. Excitement of the muscle causes an electrical impulse to be generated within the muscle; the impulse is transferred to the recording electrodes, thereby providing an indication to the surgeon as to the location of the nerve. Stimulation is accomplished using hand held monopolar or bipolar probes such as the Electrical Stimulus Probe disclosed in U.S. Pat. No. 4,892,105 (to Richard L. Prass), the entire disclosure of which is incorporated herein by reference. The probe of Pat. No. 4,892,105 has become known as the Prass Flush-Tip Monopolar Probe and is insulated up to the distal tip to minimize current shunting through undesired paths. An improved structure for a bipolar probe is disclosed in the provisional patent application entitled Bipolar Electrical Stimulus Probe (filed Aug. 12, 1998, application No. 60/096,243), the entire disclosure of which is also incorporated herein by reference.
Another method of nerve localization involves mechanical stimulation of the nerve of interest by various dissecting instruments. Direct physical manipulation of a motor nerve may cause the nerve to conduct a nerve impulse to its associated musculature. If those muscles are being monitored using a nerve integrity monitoring instrument, the surgeon will hear an acoustic representation of the muscle response in close temporal relationship to the antecedent mechanical stimulation. This will allow the nerve of interest to be roughly localized at the contact surface of the dissecting instrument.
Prior art nerve integrity monitoring instruments (such as the Xomed® NIM-2® XL Nerve Integrity Monitor, manufactured by the assignee of the present invention) have proven to be effective for performing the basic functions associated with nerve integrity monitoring such as recording EMG activity from muscles innervated by an affected nerve and alerting a surgeon when the affected nerve is activated by application of a stimulus signal, but have significant limitations for some surgical applications and in some operating room environments.
A first problem is users have noticed certain EMG measurement artifacts have a disruptive effect on monitoring and tend to cause undesirable false alarms. In particular, EMG monitoring often is performed during electrocautery in a surgical procedure, wherein powerful currents surge through and cauterize the tissue, often to devastating effect on the monitor's sensitive amplifier circuits. Electrocautery can also induce an undesired direct current (DC) offset from buildup of charge on the monitoring or sensing electrodes or within recording amplifier circuitry. A method of muting during periods of electrocautery using in-line detection of electrocautery, based upon frequency and amplitude was disclosed in Prass, et al.: “Acoustic (Loudspeaker) Facial Electromyographic Monitoring: Evoked Electromyographic Activity”, Neurosurgery 19: 392–400, 1986; and an improved method involving an inductive probe pickup was described in U.S. Pat. No. 4,934,377, entitled “Intraoperative Neuroelectro-physiological Monitoring System”, by Prass, et al., the entire disclosures of which are incorporated herein by reference.
Brief pop noise in the form of high frequency bursts (caused by spurious electromagnetic and current artifacts or when non-insulated metal instruments are accidentally brought into physical contact) may be recorded during nerve integrity monitoring. These brief artifacts may be confused for true electromyographic (muscle) responses and may lead to misinterpretation and false alarms, thereby reducing user confidence and satisfaction in nerve integrity monitoring. Maintenance of high common-mode rejection characteristics in the signal conditioning path has helped to reduce such interference, however, false alarms still occur. Any solution tending to eliminate or minimize false alarm problems would increase the accuracy and effectiveness of monitoring procedures.
Prior art nerve integrity monitoring devices incorporate a simple threshold detection method to identify significant electrical events based upon the amplitude of the signal voltage observed in the monitoring electrodes, relative to a baseline of electrical silence, a methodology having disadvantages for intraoperative nerve integrity monitoring. Use of intramuscular electrodes in close bipolar arrangement (as described in U.S. Pat. No. 5,161,533, cited above) provides adequate spatial selectivity and maintenance of high common mode rejection characteristics in the signal conditioning pathway for reduced interference by electromagnetic artifacts, but yield a compressed dynamic range of electrical voltage observed between the paired electrodes. When physically situated near one of the electrodes, a single nerve motor unit (e.g., activation of a single nerve fiber) may cause an adequate voltage deflection to be heard (by a surgeon listening to the EMG audio signal feedback) as a clear signal spike or exceeding a predetermined voltage threshold. Moreover, with close electrode spacing and bipolar amplification, recording of larger responses is frequently associated with internal signal cancellation, significantly reducing the amplitude of the observed electrical signal. The resultant compressed dynamic range is advantageous for supplying direct or raw EMG audio signal feedback to the operating surgeon, in that both large and small signal events may be clearly and comfortably heard at one volume setting, but an EMG audio signal feedback having compressed dynamic range provides limited ability to fractionate responses based upon magnitude of the response or obtain an accurate measure of signal power. Another disadvantage of prior art methodology of threshold detection is that the surgeon cannot readily distinguish or select between electrical artifacts and EMG activity.
A second problem is that the nerves of interest may frequently exhibit a variable amount of irritability during the surgical procedure, which may be caused by a disease process or by surgical manipulations such as mild traction or by drying or thermal effects. Such nerve irritability is recorded by nerve integrity monitoring electrodes and is displayed and annunciated to the operating surgeon as a series of “beeps” caused by repetitive triggering of threshold detection or by repetitive electromyographic spikes. Because nerve irritability does not appear in close temporal relationship to particular surgical manipulations, it provides no localizing information. When such repetitive activity is observed, the surgeon usually ceases all ongoing surgical manipulations and may irrigate the surgical field in an attempt to reduce nerve irritability. Once a reasonable effort to reduce nerve irritability has been carried out, any residual nerve irritability becomes “noise” and may interfere with the ability to detect electrically and mechanically stimulated nerve activity. Any methods to reduce the effect of background nerve irritability on detection of brief bursts of nerve activity would enhance localization of nerves of interest during periods of increased nerve irritability.
A third problem arises when monopolar probes, bipolar probes or electrified instruments are selected for electrical stimulation during intraoperative neurophysiological monitoring. Each type of probe has its own advantages, disadvantages and “best application” during intraoperative procedures. Because of a variable tendency for current shunting, the optimum stimulus intensity may vary significantly among probes. For a given probe type, the ideal stimulus intensity is low enough to allow spatial selectivity, but high enough to avoid false-negative stimulation as a result of current-shunting or other influences. The commercial EMG-type nerve monitors of the prior art have a single current-source terminating in either one or two outputs. If there are two outputs, the outputs are connected in parallel with a single common stimulus intensity setting and so there is no ability to provide separate (optimized) stimulus intensities or to guard against parallel communication between the two outputs. If both outputs are connected to stimulus instruments, undetected current-leak could occur through parallel channels and result in false-negative stimulation. At least one manufacturer or prior art monitoring instruments offers a switchable connector at the stimulus probe terminus, allowing more than one stimulus instrument to be kept in readiness, and avoiding parallel connections to the unused instruments, but performing the act of switching requires a surgical staff member such as a nurse or technician and so is cumbersome and, being time consuming, expensive.
A related problem is that prolonged nerve irritability may be due to light anesthesia, rather than to inherent nerve irritability. Any method to distinguish these two possibilities would enhance interpretation during nerve integrity monitoring.
Another problem confronting users of prior art nerve integrity monitoring devices is that quantative measurements of nerve function are relatively cumbersome to obtain, since equipment setting changes must be performed by operating room personnel while electrical stimulation procedures are performed by the operating surgeon. For example, a threshold determination for electrical nerve stimulation is an accepted indication of functional nerve integrity. Determination of response threshold requires stimulation at multiple stimulus intensities, which must be changed manually, and nerve responses must be recorded at each stimulus intensity level. With prior art technology, this process is time-intensive and discourages serial determinations during the operation as an ongoing measure of nerve integrity. Threshold determinations are typically performed only at the end of the operative procedure as a prediction of immediate postoperative function. When using prior art methods, if the threshold is found to be abnormal, the surgeon is usually unaware of when the change to abnormality occurred during the operative procedure. Any method making quantitative measurements of nerve function convenient and rapid to obtain would enhance nerve integrity monitoring.
Another concern is how functions are controlled. There is a relatively strong conceptual separation between off-line control (performed at some time other than during the procedure) and on-line control (performed during a surgical procedure), as pertains to control of intraoperative neurophysiological monitoring system functions through the use of input devices. “Off-line” operations are performed when monitoring is not actively being performed, for example, as when logging-in patient information, setting system preferences or retrieving saved-data for “post-production” analysis, whereas “on-line” refers to periods of active intraoperative neurophysiological monitoring.
In prior art nerve integrity monitoring devices, controls for off-line functions consist of front panel knobs and switches or keyboard and mouse with proprietary software to perform common setup functions and parameter adjustments. Additional back panel switches may be available to adjust less commonly changed parameters, such as stimulus rate and duration. For multi-channel nerve integrity monitoring with qualitative and quantitative signal analysis, front and back panel hardware is cumbersome and too limited in scope. Greater flexibility and convenience in off-line controls is available through use of keyboard and mouse input and software capabilities to modify and store setup information in archival files for facilitation of off-line setup functions. A limitation of prior art strategies is that the setup information is held in volatile memory during actual monitoring operations, rendering the setup information vulnerable to strong electrical surges, electromagnetic noise or accidental power interruptions. An electrical surge or accidental unplugging may cause loss of all new (different from “default”) setup information, requiring a “reboot” of the system and adjustment to get back to the desired settings. Any method for off-line control allowing similar flexibility a to keyboard and mouse input and having the convenience of designated software with archival (file) storage of setup information, but without risk of erasure by spurious electrical events or accidental equipment unplugging, would represent a significant advance for nerve integrity monitoring. Stimulation devices of the prior art for neurophysiological monitoring are manually controlled through front panel potentiometers and switches or with mouse and keyboard to produce paired or burst stimuli and stimuli of opposite polarity in an alternating pattern, but lack the ability to deliver consecutive stimuli of differing intensities or alter the pattern of stimulation at a predetermined time without that time consuming manual input. Analogously, none of the monitoring instruments of the prior art provide delivery of selected stimuli in coordination with data acquisition, analysis, display, and storage. Moreover, in prior art nerve integrity devices, control of on-line functions is performed by keyboard and mouse or by front panel controls and, because of a possible breach of sterility, the operating surgeon cannot perform such functions by himself or herself and so changing equipment settings requires involvement of hospital personnel at the request of the operating surgeon and may be time-consuming, cumbersome and possibly risky, since the changed settings may be inaccurate. Any method allowing rapid and accurate changes in equipment function without the need of ancillary operating room personnel and without risk to maintenance of sterility would be considered an enhancement of nerve integrity monitoring.
An important function of intraoperative neurophysiological monitoring is detecting brief episodes of EMG activity, caused by direct electrical and mechanical stimulation. Detection allows the surgeon to localize a nerve of interest approximately at the contact surface of the dissecting or stimulating instrument. Detection of brief, localizing EMG activity is frequently obscured by the presence of repetitive EMG activity caused by “baseline” nerve irritability. Such irritability may be due to nerve compromise caused by the disease process itself or to various surgical manipulations, such as mild traction, drying, thermal stimulation, or chemical irritation. When significant repetitive activity is observed, the surgeon typically ceases all surgical manipulations and may irrigate the wound in an attempt to “quiet” nerve irritability. Once a reasonable attempt has been made to allow the nerve to become quieted, any remaining repetitive activity is essentially “noise” and may interfere with hearing more important brief EMG responses that allow localization of the nerve of interest. Such background irritability is particularly a problem during acoustic neuroma resections, which is one of the most common procedures for which facial nerve monitoring is used.
Redundancy afforded by multi-channel monitoring of (single) nerves of interest provides some opportunity to maximize the ability to detect localizing information during periods of problematic repetitive (non-localizing) activity. The most common application of nerve integrity monitoring involves monitoring the facial nerve. The facial nerve has a long course, beginning in the cranial cavity, then through a bony channel (fallopian canal) within the temporal (ear) bone, exiting behind the ear to swing forward and innervate the nerves of the facial expression. The nerve is at risk during a number of surgical procedures involving the ear, the temporal bone and intracranially. Intracranially, and in its course through the temporal bone, the nerve appears as a single nerve bundle, with no internal topographical organization. As the nerve exits the temporal bone behind the ear it finally separates into two major trunks, which further divide into 5 major branches. Multi-channel nerve integrity monitoring of the facial nerve involves placing electrodes into multiple facial muscles, representing multiple branches of the nerve. While not necessarily the preferred approach, the lack of topographical organization of the intracranial and intratemporal portions of the facial nerve, allows monitoring during removal of acoustic neuromas and during ear surgery with only one or two electromyographic channels.
Multichannel monitoring of the facial nerve is preferred in order to increase sensitivity and to provide redundancy in the event of electrode failure. Redundant facial nerve monitoring channels also provides flexibility to maximize the ability to detect localizing, brief non-repetitive EMG activity. The upper and lower facial musculature have been observed to have differential tendencies to exhibit mechanically evoked EMG activity. The lower face tends to be more sensitive in eliciting mechanically-stimulated EMG activity but also has a greater tendency to exhibit “background” nerve irritability. During periods when background repetitive EMG activity obscures auditory detection of more important and localizing non-repetitive activity, the most active EMG channels can be deleted (muted) from the signal directed to the surgeon through audio loudspeaker(s). The remaining EMG channels, having less competing background noise to interfere, are more easily heard by the operating surgeon in order to detect (localizing) mechanically and electrically stimulated EMG activity.
The majority of prior art nerve integrity monitoring devices have only two channels, which allows little redundancy and flexibility. When repetitive activity becomes bothersome and persistent, despite reasonable efforts on the part of the operating surgeon to allow the nerve to quiet down, the surgeon may ask an operating room employee to “turn the monitor down.” This solution is problematic, because it may cause the surgeon to miss hearing important localizing EMG information. Alternatively, with the availability of multiple (redundant) EMG channels, a nurse or operating room technician may individually eliminate each electrode channel in an attempt to identify the offending channels, so that they may be (temporarily) eliminated. This process may be greatly facilitated, if there is some visual indication of relative EMG activity among the various EMG channels. However, even with visual displays, the process may still be time consuming and, therefore, expensive. Moreover, once certain “offending” channels have been muted, there may be long periods before the surgeon, the nurse, or operating room technician remember or “feel safe” to add these channels back to the audio signal. This may cause unnecessarily long periods of decreased sensitivity.
There is a need, then, for a nerve integrity monitoring instrument having greater flexibility and stability in use, greater sensitivity and specificity (e.g., noise rejection and artifact identification), and a user interface more readily adapted to performing the monitoring procedures required without distraction to the surgeon while concentrating on the medical aspects of the surgical procedure.